1. Technical Field
The present invention relates to controlling fluid flow at a flow control location, and is particularly directed to an apparatus for preventing undesired fluid flow past a flow control location defined by a nip formed by a pair of rollers. The apparatus of the present invention is especially applicable to a dampening system of a printing press.
2. Background Art
A dampening system for a printing press typically includes a plurality of rollers for transferring dampening fluid to a printing plate during operation of the printing press. The printing plate is secured to a plate cylinder of the printing press. One of the plurality of rollers is a pan roller rotatable about its longitudinal central axis. Another one of the plurality of rollers is a slip roller rotatable about its longitudinal central axis and located adjacent to the pan roller. A nip is formed between the two rollers along the axial extent of the two rollers. The nip has a fluid entrance side at which fluid enters the nip and a fluid exit side from which fluid flows from the nip.
During operation of the printing press, the pan roller rotates about its longitudinal central axis in one direction while the slip roller rotates about its longitudinal central axis in the opposite direction. Thus, at the nip between the two rollers, the pan and slip rollers rotate in the same direction. The pan roller is partially immersed in a supply of dampening fluid, and the dampening fluid adheres to the outer surface of the pan roller as the pan roller rotates. The fluid is carried on the outer surface of the pan roller to the entrance side of the nip. Thus, the fluid enters the nip between the two rollers.
When the dampening fluid carried on the outer surface of the pan roller enters the nip between the two rollers, some of the fluid is transferred onto the outer surface of the slip roller. The fluid not transferred onto the outer surface of the slip roller remains adhered to the outer surface of the pan roller. The fluid adhering to the outer surface of the slip roller is subsequently transferred by other rollers onto the outer surface of the printing plate. The fluid which remains adhered to the outer surface of the pan roller is carried back to the supply of dampening fluid.
As known in the art, it is desirable to control the amount of fluid transferred to the outer surface of the printing plate. One way to control the amount of fluid transferred to the printing plate is to control the rotational speed of the pan roller and the rotational speed of the slip roller. An increase in the speed of each of the rollers increases the amount of fluid transferred to the printing plate. Likewise, a decrease in speed of each of the rollers decreases the amount of fluid transferred to the printing plate. Another way to control the amount of fluid transferred to the printing plate is to skew one of the two rollers along the axial extent of the two rollers. Still another way is to increase or decrease the pressure between the two rollers at the nip. Thus, the nip is a flow control location in the dampening system.
If the amount of fluid carried on the outer surface of the pan roller to the entrance side of the nip exceeds the amount of fluid flowing out of the nip at the exit side of the nip, a buildup of excess fluid at the entrance side of the nip occurs. The excess fluid at the entrance side of the nip tends to flow to the opposite axial ends of the nip. Although some of the excess fluid drips from the opposite axial ends of the pan roller back into the fluid pan due to gravity, some of the excess fluid may flow around the opposite axial ends of the nip (the flow control location) and onto the slip roller. Some of the fluid transferred onto the slip roller in this manner is subsequently transferred to the printing plate. This fluid flow around the opposite axial ends of the nip onto the slip roller is undesirable because such flow is uncontrolled and unintended.